The invention is concerned with a pressure filtration apparatus in which pressurized gas is utilized to both seal the filtration unit and to drive the material to be filtered through a filter in the filtration unit.
Various types of pressure filtration systems are well-known in the prior art. Systems such as disclosed, for example, in U.S. Pat. No. 3,893,920 employ a cannister having two chambers separated by a permeable membrane or filter. In these systems, a fluid to be filtered is placed in one chamber and a source of negative pressure is applied to the other chamber to draw the liquid through the filter. These devices have many disadvantages, notably that in order to obtain a tight seal between the vacuum source and the filtration container, it is necessary to manipulate the vacuum connection of the filter unit and thus to expose it and its contents to the possibility of contamination. Other major disadvantages of the prior art devices include slowness of filtration and foaming of the filtrate. Another important disadvantage of the negative pressure devices of the prior art resides in the fact that if it is desired to eliminate the above disadvantages by using them as positive pressure devices, the sealing means for the filtration chamber is subjected to pressures which tend to push the sealing means out of the upper chamber. To compensate, extraordinary pressure must be used to seal the unit which causes jammed sealing means after completion of a filtering operation.
Positive pressure filtration devices are also known in the art, for example, U.S. Pat. No. 3,493,496 teaches an impermeable, flexible diaphragm which transmits pressure from a driving gas or fluid to the liquid to be filtered. A mechanical piston is disclosed in U.S. Pat. No. 3,565,256 to communicate pressure from a driving gas (Freon) to the liquid. While it is also known in the art that generally faster filtration rates, with less foaming of fluids, may be accomplished with positive pressure filtration devices, one of the major difficulties to successful filtration-using positive pressure has been the inability to obtain a tight and easily releasable seal of the filtration cannister while avoiding contamination of the material to be filtered. One of the proposed sealing methods is disclosed, for example, in U.S. Pat. No. 3,540,857 wherein a ram or piston pushes the filtration cannister into sealing contact with an anvil having a central opening. The pressure applied by the piston is mechanical and is maintained at a preset value. Unfortunately, in such devices, if the positive pressure supplied to the fluid contained in the filtration cannister should, even momentarily, exceed the preset sealing pressure applied by the piston, the seal will be broken. On the other hand, if the seal is not prematurely broken, such seals have proven difficult to release after filtration.
Various types of sealing arrangements for negative pressure filtration systems are also well-known in the art. In U.S. Pat. No. 2,874,843 a negative pressure is applied to a filtrate receptacle to cause a downward force to seal a filter unit to the receptacle. U.S. Pat. Nos. 2,896,787 and 3,782,175 disclose sealing arrangements wherein an apertured, circular piece of rubber or a plastic ring overlay a filter. A vacuum applied to a filtrate receptacle draws the rubber or ring down over the filter, thus sealing the filter. U.S. Pat. No. 3,063,289 discloses a sealing arrangement wherein a rubber stopper having a central aperture is disposed between a filtrate receptacle and a filter chamber. A source of negative pressure seals the stopper within the filtrate receptacle and draws a fluid through a filter disposed in the filter chamber. However, such sealing arrangements are prone to jamming and require specially designed filtration units.